1. Field of the Invention
The present invention relates to a hydraulic control apparatus in a working machine such as a hydraulic excavator.
2. Description of the Related Art
Cutoff control in a hydraulic control apparatus for cutoff control of pump flow rate is performed, in a hydraulic circuit provided with a relief valve for determining a maximum pressure of the circuit, for the purpose of reducing energy loss by reducing a relief flow rate, that is, a passing flow rate of the relief valve.
With respect to techniques of performing the cutoff control, the following techniques have been known.
(1) A technique disclosed in Japanese Unexamined Patent Application Publication No. 10-246204. In a negative control, a throttle is provided in a downstream side of a relief valve. In the case that a pressure of an upstream side of the throttle increases, a pump flow rate is decreased.
(2) A technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-038536. A temperature of a relief valve is detected and in the case that the relief valve temperature increases, a pump flow rate is decreased.
(3) A technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-265002. A pressure feedback control is performed so that a pump pressure is to be a set value or below the set value.
However, according to Japanese Unexamined Patent Application Publication No. 10-246204, the throttle is provided in the downstream side of the relief valve. Then, a pressure loss due to the throttle occurs, and the desired effect in energy efficiency improvement in the entire system is not enough.
According to Japanese Unexamined Patent Application Publication No. 2002-038536, due to heat capacity of the relief valve, a time lag is generated between a generation of the relief flow rate and a temperature increase of the relief valve, the execution of the cutoff control is delayed. Accordingly, the reduction effect of the relief loss is not enough. Further, after the relief flow rate becomes zero, the remaining heat is still detected and the cutoff control is continued. Accordingly, the actuator flow rate comes short, and the driving force is reduced.
On the other hand, according to Japanese Unexamined Patent Application Publication No. 2005-265002, the pressure feedback control is performed so that the pump pressure is to be the set value (cutoff pressure) or below the set value. Then, basically, the relief flow rate can be reduced and this is effective in the energy efficiency improvement.
More particularly, in this case, between the pressure feedback control for cutoff and an ordinary control (a positive control, a negative control, a load sensing control, or the like) for controlling the pump flow rate based on operation information, a control having a smaller flow rate command value is selected. Then, the pressure feedback control is selected, and the cutoff operation is carried out.
However, the pressure feedback control aims for a smaller pressure than the set pressure of the relief valve. Accordingly, as long as the pressure feedback control is selected, the pump pressure is not increased to the maximum pressure, and the driving force comes short and result in, for example, decrease in hill-climbing ability on a sloping road.
In Japanese Unexamined Patent Application Publication No. 2005-265002, as a countermeasure for the hill-climbing ability, a tilt of a vehicle body is detected, and on a sloping road, the cutoff control, that is, the pressure feedback control, is switched to “off”.
However, in such a structure, detection means for detecting the tilt of the vehicle body and the wiring equipment for the detection means have to be newly added to the existing circuit. Accordingly, the instillation cost is increased and the application of the detection means and the wiring equipment on the existing machine is difficult.
Further, the shortage of driving force can occur not only in the hill-climbing but in various operations, however, any countermeasure to this problem has not been taken.
For example, in rotation, since a rotation motor is not immediately accelerated due to the effect of inertia of the rotating body, an inflow flow rate is greater than an outflow flow rate. Thus, the circuit pressure increases, and by the pressure feedback control performed in response to the pressure increase, the pump flow rate is controlled.
In the case that such a state is continued, the relief flow rate is reduced. However, the rotation pressure is not increased, and the rotating body can rotate in only a very slow speed or can stop without rotation. This can occur in rotation on a flat road, and the problem becomes significant in an increasing rotation at the time of rotating to the upper side on a sloping road.